Hank Fletcher

I'm trying to figure out how I can use a cordless drill to control my robot. I mentioned elsewhere that I'd hooked a drill up to my modified computer PSU, but that the PSU auto-shuts off whenever you pull the drill trigger. The drill will run if you press and hold the trigger before you plug in the supply, so there must be a momentary excessive load when you first press the trigger.

It was suggested that I use a capacitor to try and counter the PSU shut off. I've read a bit about capacitors, but I'll have to admit that what I understand about them in theory doesn't translate easily into my real world, practical understanding of electronics. What I'm trying to say is, how can I determine what type of capacitor I should use?

My (very rough) estimate is that the 14.4V drill has about a 20A spike (surge?) when you first press the trigger. So do I put the capacitor in series between the PSU's VSS and the positive terminal inside the drill handle? And again, how should I go about determining what size capacitor to use? I'm not adverse to experimenting, but I like to know a little about what I'm doing first, particulary when there are potentially dangerous charges involved.

Any hints, tips, speculation, suggestions, links, are greatly appreciated! Thanks!

Sceadwian

You put it in parallel (not series) with the VSS/GND leads. Most power switch modes 'soft start' so it shouldn't trip the power supply when it first turns on, it may though, never tried it. You could use a simple current limiting resistor in series with the motor to limit it's max current, but that will be wasteful.

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Hank Fletcher

Thanks, Sceadwian. Can you explain (or point me to where I can find) the theory of using a capacitor in that way - a lot of this stuff is still new to me. I think I know how a capacitor works, but for some reason or another, understanding how or why to use one in any given situation is still a bit of a mystery to me.

Set up like you described:
- I turn the PSU on
- the capacitor begins to charge (will it not short if it's rated over 12V?)
- I press the trigger on the drill
Then what's happening? I just would like to understand is all. I don't understand how I put the capacitor in parallel without shorting it, either. Any chance of a quick schematic, just to make sure I'm understanding you correctly?

gootee

If you connect a large electrolytic capacitor from the power supply output that your drill will use, to the power supply's ground (note the correct polarity for an electrolytic, and use one with a voltage rating that's higher than your supply voltage), the cap will already be charged when you turn on the drill. The capacitor's stored charge will help supply any current surge that the drill demands from the supply. If the capacitor was large-enough, and could respond fast-enough, the supply might not see the surge demand at all.

If you have trouble getting it to work, you might want to try using several large-ish capacitors in parallel, instead of just one. In this type of testing, I don't think some inital amount of overkill could be a bad thing (unless you get to the point where the capacitors' demands during charging cause the power supply to shut down, or be damaged). So, if you can scrounge some quite-large capacitors from the power supply of a dead monitor, or computer, or whatever, or otherwise acquire them, you could try using some tens of thousands of microfarads (uF), to start with. If you do get it to work, you can then figure out what minimum capacitance seems to be needed for reliable operation, and add some margin to that to try to make life easier for the power supply.

Also, which PSU output are you using? The 15V? I think that some computer (and some SMPS types, in general) need to have some minimum load on them, all the time, to operate correctly. It MIGHT even be the case that your drill-startup problem is more-related to THAT, rather than to trying to draw too large of a current spike.

I'm not sure that that is probable-enough to even be worth mentioning, but, in case the capacitor _doesn't_ solve your problem, then maybe you would want to also try the following.

You might be able to tell what minimum load is needed from the specs on the label, or look it up on the web, somewhere, for your make and model. At any rate, you might need to put a small power resistor on the 5V supply (or 3.3V if you have that), since that's usually the "master" one, which needs to always have a load, if your supply is one of those that does need that.

I have no idea what amount of current you might need to draw, in that case. It might be a couple percent of the 5V supply's rated current. Maybe try a resistor that would pull about 1 Amp, for a first try, since you could then use a common cement power resistor of about 5 Ohms, rated for say 10 Watts or 15 Watts (since it will dissipate 5 Watts, if it's 5 Ohms and draws 1 Amp with 5V across it, and a 5 Watt one would get very hot, then). If you get two or three of the 15 Watt ones, and it doesn't help with just one of them, you could try two in parallel, or three in parallel, to draw more current. If one works, though, then you could try using more ohms, to get less current, and less heat and waste. So then you could try putting them in series.

The currents and resistance I gave were just a totally-wild guess (hopefully on the overkill side, to raise the probability of a correct diagnosis on the first try). So if some minimum load does turn out to be required, you'd want to find the largest resistance that gave reliable operation, and would probably want to end up with one properly-sized resistor with an appropriate power rating etc, and maybe a heatsink (like the case of the PSU, possibly, if you can insulate the leads well-enough).

Good luck.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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Hank Fletcher

Thanks, gootee! I'm not sure about opening up a computer monitor - I'm a bit too much of a newb, and I've heard there's some potential for dangerous shocks that way. Maybe I'll see if I can pick up something appropriate from a store.

Thanks for the tips. I already had a fairly lengthy discussion about this in another thread, because I was having trouble with a 10 ohm, 10W resistor overheating. In the end, I chose to use a 1157 auto lamp instead, and that seems to work just fine. Still, any sudden load over a couple amps will cause the PSU to shut down (even with the constant load on the 5V rail), hence the reason I started this thread.

Sceadwian

It's the sudden load that's the problem. PC PSU's work based on the increase in current to trigger it's safety off, basically meaning you can go from 1amp to 20 amps just fine, but if you do it too fast the power supply trips it's safety shut off. If you place a heavy power resistor in series with the positive power lead of the power supply, and the capacitor/capacitor bank in parallel on the other side (toward the load) of it the power resistor will limit the maximum current by brute force (wasting some as heat) if the capacitor is charged though It can provide huge amounts of pulse currents to the load side for short periods for starting your motor, while it will gradually charge back up to working voltage through the resistor.

If you need a schematic it's pretty simple.
http://en.wikipedia.org/wiki/Low_pass_filter
The hard part is the load resistor as to be useful it needs to be relatively high to limit rush current but it will also cause a voltage drop. I know of some PSU's that have external 3.3V sense lines but none that have 12 volt sense lines so the resistor is going to drop voltage.

If you get past though point where you can use a low pass filter like that to avoid the trip, you probably need to learn how to design your own power supply.

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gootee

Yes, that all sounds exactly right. (Sorry, Hank! I should have suggested that! I must not have gotten enough sleep, the night before.)

I've used thermistors, similarly, just before the filter cap in a power supply, for example. Unfortunately, a thermistor probably wouldn't work, in this case, because it wouldn't cool fast enough to prevent the problem if the drill was turned off and back on.

The minimum resistor value and power rating could be calculated, knowing the currents and the capacitor's specs. But I guess if he's using something over 10000 uF, he could just experiment with resistances from 0.5 to 5 or 10 Ohms, as a rough guess, and go up or down from there until it worked.

The resistor's power rating should only need to cover the steady-state case, since the surge should be short-enough to not worry too much about it, although some resistors' data sheets do give the data that would be needed to make sure it was also OK for the surge.

He should use the smallest resistor that will solve the problem. And that will also have the lowest required power rating.

There's one thing I still don't understand: Why did it work OK when he started the PSU with the drill already turned on? Is it maybe just a "feature" of the PSU's protection circuitry?

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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Sceadwian

Should be short enough but you can't garuntee that during a fault, otherwise you might as well disable the DV/DT threshold completly and just fuse it.

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